Basal-like breast cancer (BBC) is a subtype of breast cancer with poor prognosis. Although germline mutations of the BRCA1 gene are a major cause of basal-like breast cancer, there is increasing evidence suggesting that down-regulation of PTEN levels and p53 mutations are frequently associated with basal-like breast cancers. Thus, functional deregulation of both p53 and PTEN activities is critical for tumorigenesis of basal-breast cancer. The p53 tumor suppressor is a short-lived protein whose stability is mainly controlled by the proto-oncoprotein Mdm2 through ubiquitination. During the last period of this funded project, we identified HAUSP as a novel regulator involved in p53 activation and demonstrated a dynamic role of the HAUSP deubiquitinase in regulating the p53/Mdm2 pathway. Surprisingly, by a completely independent approach, we have discovered that HAUSP also plays an important role in regulating PTEN function. The PTEN tumor suppressor is a lipid phosphatase that plays a central role in regulating Phosphatidylinositol-3-kinase (PI3K) signal transduction cascade. Recent studies indicate that the function of PTEN is tightly regulated by ubiquitination. To elucidate the role of revisable ubiquitination in PTEN regulation, we have now constructed a library expressing the majority of human Dub family proteins. By screening ubiquitinated PTEN as a substrate with this library, we have identified HAUSP as a deubiquitinase of the PTEN protein. Moreover, our preliminary studies also indicate that HAUSP-mediated deubiquitination of PTEN significantly is required for its major biological function in vivo such as inhibiting AKT activation. Consistent with this finding, the levels of HAUSP are found to be down-regulated in basal-like breast tumors. The central hypothesis of this project is to test whether HAUSP is crucial for regulating PTEN function in vivo and that its down-regulation is critically involved in the tumorigenesis of basal-like breast cancers. In Aim 1, we will investigate the role of HAUSPmediated deubiquitination in modulating the biochemical functions of PTEN in basal-like breast cells. In Aim 2, we will examine the role of HAUSP-mediated effect on PTEN in the development of basal-like breast cancer by using mouse models.